Scotch boilers and triple-expansion endurance

Water-tube boilers offered greater performance, and early consideration of them fitted the Royal Navy's effort to expand supplier capacity for destroyers and cruisers. As war approached, however, waiting for those manufacturers would have delayed the escort program. Scotch marine boilers and triple-expansion engines could be produced, operated, and repaired across the merchant-shipping world, with reservists already familiar with the machinery. They also gave the corvette useful endurance at convoy speed rather than the high power demanded by fleet units. The tradeoff was vulnerability: two boilers drove one engine, shaft, and screw, so damage to the engine or shaft could disable propulsion even if a boiler remained usable.